This invention relates to sensitive, fast responding fluid measuring apparatus and, more particularly, to an improved hot wire anemometer.
There are two common types of temperature sensors employed in anemometers and chromatography to measure gas characteristics. One type is a thermistor bead having a negative temperature coefficient of resistance. Although high sensitivity can be achieved with a thermistor bead, its response to changes in the gas characteristics is slow due to the relatively large mass of the bead. The other type is an elongated thin electrically conductive wire, called a hot wire, which has a positive temperature coefficient of resistance. Although a hot wire responds quickly to changes in the gas characteristics, it does not generally permit as high a sensitivity as a thermistor bead.
In a hot wire anemometer, the hot wire is connected to serve as one branch of an electrical bridge circuit. Current passing through the hot wire heats the wire, thereby increasing its resistance. The hot wire is disposed in an elongated cavity through which the gas to be measured flows and cools the hot wire accordingly. If the type of gas passing through the cavity is known, the resistance change of the hot wire is a measure of the gas flow rate. If the flow rate of the gas passing through the cavity is unknown, the resistance change of the hot wire is a measure of the thermal conductivity of the gas and, hence, the gas type.
The longer the hot wire for a given volume of the hot wire cavity or the smaller the volume of the hot wire cavity for a given hot wire length, the greater is the cooling effect per unit of gas flow through the cavity. Therefore, to achieve high sensitivity, the objective is to provide a large ratio of hot wire length to cavity volume. The factors limiting this objective are the restrictions on the overall size of the apparatus, the ability to bore a long, straight cavity having a small cross section in a piece of material, and support for the hot wire so it remains out of contact with the sides of the cavity.